1. Field of the Invention
The present invention relates to a control unit for use with a machine having main spindles for workpiece rotation and tool rotation and provided with a tapping function, and more particularly, to a control unit for controlling the drive of main spindles so as to achieve tapping onto an end face of a rotating workpiece.
2. Description of the Related Art
In a machine of a type having main spindles respectively for workpiece rotation and for tool rotation, e.g., a compound lathe such as a turning center, a series of machining operations is implemented in succession. For instance, tapping is made onto an end face of a workpiece after cutting onto the peripheral face of the workpiece is performed, and then another cutting operation is carried out. In this case, generally, upon completion of the first cutting operation, the rotation of the workpiece is decelerated and stopped, and then the tapping is started. Further, upon completion of the tapping operation, the rotation of the workpiece is started and is then accelerated until a predetermined rotational speed suited for the second cutting operation is reached. Conventionally, therefore, it is impossible to effect machining which requires workpiece rotation simultaneously with tapping. Moreover, the workpiece rotation must be decelerated and stopped before the start of tapping, and the workpiece rotation must be restarted and accelerated after completion of tapping. As a result, a total machining time required for a series of machining operations is prolonged, leading to lower machining efficiency.